An international investigation is under way over what happened to the ship
A group that hijacked the Arctic Sea threatened to blow up the cargo ship if its demand for a ransom was not met, Russian defence ministry officials say.
The vessel's eight alleged hijackers and its 15-man crew were questioned aboard a Russian naval ship off the coast of West Africa.
The Arctic Sea was said to have gone missing on 30 July as it rounded the north-western corner of France.
But Maltese maritime authorities have said it "never disappeared".
Speculation about what happened to the ship has included suggestions of piracy, a mafia dispute, a commercial row, smuggling or trafficking.
Correspondents say that despite the details given by Russian officials, the case is still laden with mysteries.
Carrying timber reportedly worth $1.8m (£1.1m), the 4,000-tonne Arctic Sea sailed from Finland and had been scheduled to dock in the Algerian port of Bejaia on 4 August.
Russian Defence Minister Anatoly Serdyukov says the hijackers attacked on 24 July.
Once on board, they threatened the crew with guns and forced them to turn off navigational and tracking equipment and sail south, the defence minister said.
"Crewmembers confirm that the hijackers demanded a ransom and threatened to blow the ship up if their orders were not obeyed," an unnamed Russian defence ministry official was quoted as saying by Interfax news agency.
Earlier reports said assailants had left the ship after 12 hours.
Four of the suspects are Estonian, two are Latvians and two Russians.
Moscow deployed vessels from its Atlantic fleet to find the Arctic Sea last week.
Russia said it reached the Maltese-flagged ship on Monday, 300 miles (480 km) off Cape Verde in the Atlantic Ocean.
The hijackers were armed but abandoned their weapons when stopped, Russian officials say.
But the Malta Maritime Authority said the Arctic Sea had been "continuously tracked" from the moment it was reported to have been hijacked until the Russian navy said it had taken the ship on Monday.
Maritime officials in Malta, Finland and Sweden had not wanted "to jeopardise the life and safety of the persons on board and the integrity of the ship", it said.
Last weekend, a multinational investigation was launched after police in Finland said a ransom demand had been made, while emphasising that they could not confirm its authenticity.